Generally as ballpoint pens, those which have a good feeling of writing, are excellent in handwriting dryness, have no bleeding and no show through in handwriting, are highly sharp in handwriting, chemically stable and have less changes with time are desirable.
In accordance with requests of feeling of writing and handwriting dryness, an ink viscosity at 20° C. is set in the range of 10,000 to 30,000 mPa·s in many of conventional ink compositions for oil-based ballpoint pens. This is higher than the ink viscosity of the ink for water-based ballpoint pens. When the ink viscosity is high in the oil-based ballpoint pen, a ball rotational resistance itself at writing becomes large and the ink condition is very heavy, which is not favorable. On the contrary, when the ink viscosity is excessively lowered, inconveniences that the ink is spilled out and the handwriting dryness worsens occur. Thus, the ink viscosity has been set in the above range.
As a method of enhancing sharpness of the handwriting, a method of increasing an amount of ink to be jetted from a pen nib, a method of increasing a content of a coloring material and the like have been proposed.
However, the ink viscosity of the oil-based ballpoint pen is higher than that of the water-based ballpoint pen, and the amount of ink to be jetted from the pen nib is small. This is because their ink jetting mechanisms are different, i.e., in the ink jetting mechanism of the oil-based ballpoint pen, the handwriting is made by rotating a ball at the pen nib and transferring the ink adhered to the ball onto a writing surface whereas in the ink jetting mechanism of the water-based ballpoint pen, the handwriting is made by migrating the ink onto the writing surface by a capillary/permeation action in the context of the ball rotation at the pen nib.
Therefore, it is not easy to increase the amount of ink to be jetted in the oil-based ballpoint pen compared to the water-based ballpoint pen.
When the method of increasing the content of the coloring material is employed, the inconveniences that ink fluidity is reduced and the ink becomes unstable with time to precipitate the coloring material occur.
When solving the above problems and further improving various required properties, the present inventor has been led to focus on effectiveness of contained water in an ink composition for an oil-based ballpoint pen containing the water.
Recently, ink compositions for oil-based ballpoint pens containing water at a small amount have been proposed. In JP 2000-256605A, an oil-based ballpoint pen ink containing water has been proposed for the purpose of making it difficult to cause unevenness of line density due to directionality when writing with a ballpoint pen. In JP 2001-311032A, an oil-based ballpoint pen ink containing water has been proposed for the purpose of providing the ink whose designed quality is difficult to change, i.e., the ink wherein the ink viscosity is difficult to reduce due to moisture absorption even when the ballpoint pen is stored for a long time, therefore, ink dropping at writing or leakage of ink at a downward tip is unlikely caused.
In the inventions described in these publications, benzyl alcohol and phenyl glycol are used. Benzyl alcohol and phenyl alcohol are well-known as organic solvents with high boiling point, whose vapor pressure is 0.1 mmHg or less.
According to a study by the present inventor, when water and only the organic solvents typified by benzyl alcohol and phenyl glycol with high boiling point, whose vapor pressure is 0.1 mmHg or less were used as the solvents, it has been shown that although dissolution and diffusion of the ink composition other than the solvents can be kept to be stable, an inconvenience occurs when intending to enhance sharpness of the handwriting by increasing the ink amount to be jetted from the pen nib. That is, when intending to enhance sharpness of the handwriting by increasing the ink amount to be jetted from the pen nib, an evaporation rate of the ink transferred on the writing surface becomes slow. Thus, it has been shown that problems such as poor handwriting dryness, bleeding to paper surface and show through occur. Therefore, according to the inventions described in the above publications, it is difficult to enhance the handwriting sharpness without impairing the handwriting dryness and inhibitory properties for the bleeding and the show through.
Furthermore, the present inventor has found after the study that a permeable performance for the paper surface is controlled by containing water at a certain amount or more.
The present inventor has also found that the contained water is effective for facilitation of the evaporation rate of the ink solvent in the handwriting and inhibition of the bleeding and the show through of the handwriting.
Meanwhile, the present inventor has found that the permeable performance of the ink can not be controlled by the contained water and a facilitation effect of the handwriting dryness and inhibitory effects of the bleeding and the show through can not be sufficiently obtained because dissolution stability of the water can not be obtained and the amount of water to be contained can not be increased to an adequate ratio when the water and only the organic solvent with high boiling point, whose vapor pressure is 0.1 mmHg or less are used as the solvents.
A object of the present invention is to provide a chemically stable ink composition for an oil-based ballpoint pen, which exerts effectiveness of contained water for various required properties, is highly sharp in handwriting, excellent in handwriting dryness, has no bleeding and no show through in handwriting and has a good feeling of writing.